A METHOD FOR IMPRINTING MICROPATTERNS ON A SUBSTRATE OF AN ORGANIC POLYMER
A method for nanoimprinting a pattern on an organic polymer substrate, comprising: (a) preparing a soft operational mold, the operational mold comprising a pattern to be replicated to the substrate; (b) soaking the operational mold in a solvent to produce diffusion of solvent to the mold; (c) removing the operational mold from the solvent, and placing it on a surface of the organic polymer substrate to form a structure, and simultaneously (i) heating the structure to a temperature T<Tg, where Tg is the glass transition temperature of the organic polymer; and (ii) applying controlled pressure in a range of 20-300 psi on the mold to effect a penetration into the surface of the organic polymer substrate, thereby to replicate the pattern of the mold to the surface of the substrate; and (d) separating the operational mold from the patterned substrate.
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The invention relates to the field of micro-imprinting. More specifically, the invention relates to a method for imprinting micropatterns on a flat or curved organic polymer body.
BACKGROUND OF THE INVENTIONNanoimprint lithography is a widely used technique for shaping in a nano-scale (or micro-scale) body surfaces, such as optical components, electronic devices, photonic nanostructures, etc. Soft nanoimprinting is a versatile, high-throughput, and cost-effective nanolithography technique in which a nano-scale pattern is mechanically transferred onto a resist by use of an elastomeric mold. Given the mechanical flexibility of soft molds, the soft imprint technique can produce high-resolution nanostructures in UV-curable polymer films deposited on substrates with unconventional geometry, such as lenses and optical fibers. However, direct patterning of thermo-formable substrates with functional nanostructures is impractical by existing techniques due to a global substrate-deformation. The nanoimprint lithography technique combines (a) nanopatterning with a resolution and minimal feature size down to single nanometers; (b) scalability and high throughput; and (c) can be performed by relatively simple and cost-effective equipment. Therefore, nanoimprint is a preferable approach for device fabrication in numerous applications, such as plastic electronics, photovoltaics, photonics, and biomimetics. Nanoimprint, in principle, can be applied to a variety of thermoplastic and UV curable resist materials, using either rigid or soft molds. A soft mold has been mostly used to pattern a non-planar surface, which is challenging for conventional lithography techniques, such as photolithography or electron-beam lithography. Ever since its introduction, nanoimprint lithography has been mainly used to produce resist-masks, whose pattern is transferred into the underlying substrate, for example, by plasma etching, or by metal-deposition followed by liftoff. However, direct patterning of functional bodies on organic polymer substrates that are commonly used, for example, in solar cells, lasers, LEDs, lenses, and facets of waveguides, have not become practical yet. Current nanoimprinting techniques can pattern an organic polymer substrate only by applying a thin film of a UV-resist, or by applying a thermoplastic resist onto a solid substrate made of inorganic material, such as silicon or glass. In the latter case, the imprinting temperature must be higher than the glass transition temperature, to allow pattern transfer from the mold to the resist. Notably, applying a resist-film onto a polymer substrate means that the patterning is performed not on the substrate itself, but rather on a layer of “foreign” material. This procedure often complicates the fabrication process and substantially limits the choice of materials. For example, (a) this technique requires the use of film material having strong adhesion to the underneath substrate; and (b) it requires the use of film and substrate materials having similar thermal expansion coefficients to avoid mechanical stresses during thermal cycles, resulting in cracks and delamination of the resist film; and (c) moreover, in optical applications there is often a necessity to match the refractive index of the imprinted film with that of the substrate, to avoid the formation of an undesired optical interface. To summarize, it would be highly preferable in many applications to directly micro-pattern the substrate's surface while avoiding the application of foreign material (such as a resist film).
Direct patterning by hot embossing of polymer substrates has been demonstrated, yet mostly for features sized in the micron scale and above. Chen et al., Soft Mold-Based Hot Embossing Process for Precision Imprinting of Optical Components on Non-Planar Surfaces, Opt. Express 2015, 23, 20977 has recently demonstrated hot embossing of curved substrates using a soft mold. However, attempts to reproduce features sized below the micron scale resulted only in partial pattern transfer. Furthermore, to achieve even a partial pattern transfer, a temperature far above the glass transition point of the embossed polymer had to be applied, resulting in a significant thermal expansion of the PDMS (polydimethylsiloxane, a type of soft silicon) mold, and as a result, a distortion of up to 15%. Still, the main constrain of hot embossing is the compromise between the pattern quality and maintenance of the substrate shape, namely, embossing at a temperature slightly above the glass transition point yields an incomplete pattern transfer while embossing at a higher temperature deforms the substrate. Such deformation is often intolerable, primarily when the substrate is used as an optical component, such as a lens.
It is an object of the present invention to provide a method for direct surface patterning of microstructures on an organic polymer substrate.
It is another object of the invention to provide a soft nanoimprint technique for patterning organic polymer bodies.
It is still another object of the invention to provide a soft nanoimprint technique for patterning organic polymer bodies, which is simple, scalable, and applied in a high-throughput manner.
It is still another object of the invention to provide a soft nanoimprint technique for patterning organic polymer bodies, having curved or flat surfaces.
It is still another object of the invention to provide a soft nanoimprint technique for patterning organic polymer bodies' surfaces to obtain an anti-reflective surface.
Other objects and advantages of the invention will become apparent as the description proceeds.
SUMMARY OF THE INVENTIONThe invention relates to a method for nanoimprinting a pattern on an organic polymer substrate, comprising: (a) preparing a soft operational mold, the operational mold comprising a pattern to be replicated to the substrate; (b) soaking the operational mold in a solvent to produce diffusion of solvent to the mold; (c) removing the operational mold from the solvent, and placing it on a surface of the organic polymer substrate to form a structure, and simultaneously (i) heating the structure to a temperature T<Tg, where Tg is the glass transition temperature of the organic polymer; and (ii) applying controlled pressure in a range of 20-300 psi on the mold to effect a penetration into the surface of the organic polymer substrate, thereby to replicate the pattern of the mold to the surface of the substrate; and (d) separating the operational mold from the patterned substrate.
In an embodiment of the invention, the operational mold is made of silicon rubber, such as PDMS.
The pattern may be imprinted, for example, on a polymer characterized by Tg>100° C., e.g., in the range from 100 to 200° C. The temperature T at which the imprinting is performed is generally at least 10° C. lower than the Tg of the polymer substrate, e.g., from 20° C. to 80° C. lower, e.g., 30° C. to 70° C. lower than the Tg of the polymer substrate. The term polymer, as used herein, includes homopolymers and copolymers. Virtually every thermoplastic polymer in commercial use contains additives. The term polymer, as used herein, of course, includes additives-incorporated polymers. According to the invention, suitable polymers to be imprinted include polyolefins (including cyclic olefins), polycarbonates, and poly(meth)acrylates.
In an embodiment of the invention, the solvent is selected from the group of (optionally substituted) aromatic hydrocarbons, e.g., benzene and substituted benzene, e.g., alkyl-substituted benzene such as toluene and xylene, or any other organic liquid capable of softening (dissolving) the organic polymer upon penetration into the surface layer of the substrate under the conditions reported herein.
In an embodiment of the invention, the heat provided to the structure is conductive, convective, or radiative heat transfer.
In an embodiment of the invention, the imprinted pattern is anti-reflective.
In an embodiment of the invention, the imprinted pattern is super-hydrophobic.
In an embodiment of the invention, the substrate is flat or curved.
In an embodiment of the invention, the polymer substrate is a polycarbonate, the operational mold is made of PDMS, the solvent loaded onto the mold is toluene, and T is from 60° C. to 90° C.
In an embodiment of the invention, the polymer substrate is a cyclic polyolefin, the operational mold is made of PDMS, the solvent loaded onto the mold is toluene and T is from 80° C. to 120° C.
In the drawings:
The invention provides a soft nanoimprint technique on a surface of an organic polymer body, either flat or curved. In another aspect, the invention provides a new nano-fabrication approach that allows a direct nanoimprint on a thermoplastic substrate surface with full pattern transfer while avoiding the deformation of the imprinted substrate shape.
Experiments and Further Discussion
To explore the miniaturization applicability of the direct-resistless nanoimprint process of the invention, the inventors created an elastomeric mold 202 patterned with various shapes whose dimensions were in the order of 20 nm. For this purpose, the inventors first fabricated a master mold by electron-beam patterning of a positive resist on a silicon substrate. The inventors then replicated a soft mold from this master mold by sequential application of hard and soft PDMS. More specifically, and as shown in
The inventors then continued to perform the procedure of
The high magnification SEM of
The new imprint approach of the invention opens a pathway to many applications unachievable to date. An important application of nanoimprint lithography, which requires features with heights of hundreds of nm and above, is moth-eye anti-reflective coating. This type of bio-inspired optical nanostructure, first discovered on the cornea of nocturnal moth Spodoptera eridania about half a century ago, is based on dense arrays of subwavelength nipples that produce a layer with an effective index gradient. Compared to traditional thin-film based anti-reflective coatings, moth-eye anti-reflective coatings are broadband, omnidirectional, and have low laser damage thresholds and better resistance to thermal shocks. Nanoimprint lithography, which combines high throughput with sub-wavelength patterning features, is ideal for fabricating moth-eye anti-reflective coating for many applications, such as solar cells. Sill, surface patterning of functional materials with a nanoimprinted moth-eye anti-reflective coating has mostly required the pattern transfer from an imprinted resist to the substrate by etching. Yet, in the case of polymeric optical surfaces, the fabrication of moth-eye anti-reflective coating could be, in principle, greatly simplified by direct nanoimprint. In such a case, there would be no necessity to cover the polymer substrate with a “foreign” material whose optical properties are different from those of the substrate. This procedure complicates the optical design and degrades its performance. Such a direct imprint of anti-reflective nanostructures, however, has not been demonstrated yet.
The inventors have directly nanoimprinted a surface of an optical polymeric substrate (Zeonex®) with a moth-eye anti-reflective coating.
The inventors first replicated a hybrid h-PDMS/PDMS operational mold from a commercial Nickel master mold patterned with moth-eye conical nanostructures (NIL Technology). Then the operational mold was used for a direct imprint. To perform the nanoimprint, the inventors first soaked two PDMS molds in toluene, then mechanically pressed them against the substrate from both of its sides using a set of mechanical clamps, and placed the pressed substrate-mold sandwich in an oven heated to 80° C. for 10 minutes. While the depth of the relief features for the PDMS mold was 200 nm, as shown in
As previously mentioned, the most significant benefit of using soft nanoimprint molds resides in their ability to pattern non-planar surfaces. To demonstrate that the invention's imprinting approach also applies to non-planar surfaces, the inventors produced a similar moth-eye anti-reflective coating on a commercial lens of optical glasses made of polycarbonate (PC), whose radius of curvature was 81 mm and 27 mm (vertically and horizontally, respectively). Here, the inventors used the similar mechanical setup previously described for Zeonex, based on clamps, to imprint the anti-reflective nanostructures on the lens's convex side.
Notably, the glass transition point of polycarbonate is about 150° C. The inventors presumed that a commercial plasticizer was added to the used polycarbonate to facilitate the lens injection molding; however, it was unknown to what extent this addition lowered the glass transition point. Still, the lens did not change its global shape at the imprinting temperature of 80° C. Again, as the imprinting mold was about 1.5 cm×1.5 cm in size, the inventors could not imprint the entire lens, but only its central part. This imprinted square region at the mold center is visible in the lens's photography because it is more transparent than the surrounding areas. The microstructural analysis of the imprinted area and the anti-reflective performance prove that the moth-eye anti-reflective coating of the invention can be imprinted on curved or flat substrates in substantially the same quality.
To summarize, the invention provides a new approach for a direct resistless nanoimprint on polymeric substrates. The procedure of the invention facilitates the surface nanostructuring of polymeric substrates. A significant advantage of this approach is that it is performed at a temperature Tsg<T<Tg, which is much lower than a temperature above Tg at which a conventional nanoimprint process is performed. Tg is the organic polymer's glass transition temperature, and Tsg is a glass transition temperature of the substrate's surface. The temperature Tsg results to be significantly lower than Tg due to the diffusion of the solvent into the mold, and therefore T can be much lower than Tg as well. The inventors have found no necessity to measure or determine the temperature Tsg to carry out the invention, as an operational temperature of T, which is lower by 20° C. to 80° suffice. This temperature is significantly lower than a temperature higher than Tg, which should have been typically used in such a process. It eliminates the possibility of the substrate's global deformation and minimizes any possible pattern distortion due to the used elastomeric mold's thermal expansion. The inventors showed herein nanoimprint of two different polymers. The invention's nanoimprint process applies to any thermoplastic (organic) polymer by selecting an appropriate solvent for each case. The versatility of the invention process and its compatibility with numerous polymer materials and substrates of arbitrary forms opens a route to many applications requiring precise scalable nanostructures of polymer surfaces.
While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many modifications, variations, and adaptations, and with the use of numerous equivalent or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.
Claims
1. A method for nanoimprinting a pattern on an organic polymer substrate, comprising:
- preparing a soft operational mold, the operational mold comprising a pattern to be replicated to the substrate;
- soaking the operational mold in a solvent to produce diffusion of solvent to the mold;
- removing the operational mold from the solvent, and placing it on a surface of the organic polymer substrate to form a structure, and simultaneously (i) heating the structure to a temperature T<Tg, where Tg is the glass transition temperature of the organic polymer, and (ii) applying controlled pressure on the mold to effect penetration to the surface of the organic polymer substrate, thereby to replicate the pattern of the mold to the surface of the substrate; and
- separating the operational mold from the patterned substrate.
2. The method of claim 1, wherein T<Tg by 20° C. to 80° C.
3. The method of claim 1, wherein the polymer is a polyolefin or a polycarbonate.
4. The method of claim 1, wherein the operational mold is made of silicon rubber.
5. The method of claim 1, wherein the solvent comprises an aromatic hydrocarbon.
6. The method of claim 5, wherein the aromatic hydrocarbon is benzene or alkyl-substituted benzene.
7. The method of claim 1, wherein the heat provided to the structure is conductive, convective, or radiative heat transfer.
8. The method of claim 1 wherein the imprinted pattern is anti-reflective.
9. The method of claim 1 wherein the imprinted pattern is super-hydrophobic.
10. The method of claim 1 wherein the substrate is flat or curved.
11. The method of claim 1, wherein the polymer substrate is a polycarbonate, the operational mold is made of PDMS, the solvent loaded onto the mold is toluene and T is from 60° C. to 90° C.
12. The method of claim 1, wherein the polymer substrate is a cyclic polyolefin, the operational mold is made of PDMS, the solvent loaded onto the mold is toluene and T is from 80° C. to 120° C.
Type: Application
Filed: Oct 15, 2020
Publication Date: Dec 22, 2022
Applicant: B.G. NEGEV TECHNOLOGIES AND APPLICATIONS LTD., AT BEN-GURION UNIVERSITY (Beer-Sheva)
Inventors: Mark SCHVARTZMAN (Tel Aviv), Maor ROSENBERG (Hod HaSharon)
Application Number: 17/770,532